Electrical Generator Protection Relays


Generator Loss of Excitation Relay (40G)

Electrical Generator is the main source of Electricity at Power Plant, we should make a many methods to protect the Generator from any faults can happen while Operation.

The next figure shows the main single line diagram and Protection Scheme for the Generator.

The Generator made by Mitsubishi Heavy Industries (MHI) and its specification as follows:

Power-------------- 440 MVA
Power Factor----- 0.85 lag
Line Voltage------ 19 KV
Speed-------------- 3000 rpm
Hydrogen (H2) Gas Cooled @ 4.5 barg.


Generator Loss of Excitation Relay (40G).

If a Synchronous machine losses the Excitation, the following condition will occur:

-When partial or complete loss of Excitation occurs on a synchronous generator, Reactive power flows from the other generator into the generator.

-The KW is controlled by the Prime-Mover input, while KVAR output is controlled by the Field Excitation. If the system is large enough to supply the deficiency in Excitation through the armature, the synchronous generator will operate as an induction generator, supplying essentially the same KW to the system as before the loss of Excitation.

-Since Synchronous generator is not designed for asynchronous operation, the machine output will oscillate slightly as the rotor oscillates in an attempt to lock into synchronism.

-Loss of Synchronism does not require immediate tripping unless there is an accompanying decrease in the terminal voltage that threatens system stability; it generally takes at least 2 to 6 Sec. to lose synchronism.

Data for setting calculations.

Generator capacity-------440000KVA
Generator Voltage--------19KVA
CT turns ratio-------------18000/5=3600
VT turns ration-----------19000/100=190
Gen. Transient impedance--------Xd’=24.8%
Gen. Synchronous impedance----Xd=195%

Relay setting calculations.
Relay Side Impedance
Z=Z% * ((KV2 * 10)/KVA) * (CT ratio/VT ratio)

Setting of small circle.
1-Circle #1 Diameter
Z=100 ((19210)/440000)*(3600/190) = 15.55 Ω

2-Circle #1 Offset
Z=24.8 ((19210)/440000)*(3600/190) = 1.93 Ω

3-Circle #1 Diameter
Setting=15 Cycles (0.3 Sec.)

Setting of large circle.
1-Circle #2 Diameter
Z=195 ((19210)/440000)*(3600/190) = 30.31 Ω

2-Circle #2 Offset
Z=24.8 ((19210)/440000)*(3600/190) = 1.93 Ω

3-Circle #1 Diameter
Setting=75 Cycles (1.5 Sec.)